Characterization of the Reversible Intersystem Crossing Dynamics of Organic Photocatalysts Using Transient Absorption Spectroscopy and Time-Resolved Fluorescence Spectroscopy

Thermally activated delayed fluorescence (TADF) emitters are molecules of interest as homogeneous organic photocatalysts (OPCs) for photoredox chemistry. Here, three classes of OPC candidates are studied in dichloromethane (DCM) or N,N-dimethylformamide (DMF) solutions, using transient absorption spectroscopy and time-resolved fluorescence spectroscopy. These OPCs are benzophenones with either carbazole (2Cz-BP and 2tCz-BP) or phenoxazine/phenothiazine (2PXZ-BP and 2PTZ-BP) appended groups and the dicyanobenzene derivative 4DP-IPN. Dual lifetimes of the S1 state populations are observed, consistent with reverse intersystem crossing (RISC) and TADF emission. Example fluorescence lifetimes in DCM are (5.18 ± 0.01) ns and (6.22 ± 1.27) μs for 2Cz-BP, (1.38 ± 0.01) ns and (0.32 ± 0.01) μs for 2PXZ-BP, and (2.97 ± 0.01) ns and (62.0 ± 5.8) μs for 4DP-IPN. From ground state bleach recoveries and time-correlated single photon counting measurements, triplet quantum yields in DCM are estimated to be 0.62 ± 0.16, 0.04 ± 0.01, and 0.83 ± 0.02 for 2Cz-BP, 2PXZ-BP, and 4DP-IPN, respectively. 4DP-IPN displays similar photophysical behavior to the previously studied OPC 4Cz-IPN. Independent of the choice of solvent, 4DP-IPN, 2Cz-BP, and 2tCz-BP are shown to be TADF emitters, whereas emission by 2PXZ-BP and 2PTZ-BP depends on the molecular environment, with TADF emission enhanced in aggregates compared to monomers. Behavior of this type is representative of aggregation-induced emission luminogens (AIEgens).


S1.1 Spectroscopy
Transient electronic absorption spectroscopy (TEAS) and transient vibrational absorption spectroscopy (TVAS) measurements of the photodynamics of five OPCs in two solvents used ultrafast laser systems at the University of Bristol and the LIFEtime facility at the STFC Rutherford Appleton Laboratory, respectively.[3] For TEAS experiments at the University of Bristol, the 800-nm output of an amplified pulsed Titanium-sapphire laser (Coherent Legend Elite HE+, 5 W, 1 kHz) was split to generate pump and probe pulses.The wavelength-tuneable UV pump pulses ( = 360 nm for all samples) were generated from the 800-nm beam using a Coherent OperA Solo optical parametric amplifier (OPA) to give pulse energies of 500 nJ at the sample.An optical delay stage in the 360-nm beam path controlled the time interval between pump and probe laser pulses, and a 500 Hz chopper was used to acquire pump-on and pump-off data in sequential laser shots.A small portion of the 800-nm output from the laser amplifier generated broadband white-light continuum (WLC) probe pulses spanning wavelengths from 350 -700 nm by focusing into a 3-mm thick CaF 2 window.WLC pulses were recollimated by an off-axis parabolic mirror and focussed into the sample.This setup allowed measurements to be recorded over time delays from ~ 100 fs to 1.3 ns.
For TVAS measurements using the LIFEtime facility, UV pump wavelengths of 360 nm and 425 nm were selected, with an average pulse energy of 300 nJ at the sample.Pairs of ~200 cm -1 bandwidth IR probe pulses, offset in central wavenumber but spatially overlapped at the sample, covered the wavenumber range 1400-1800 cm -1 .These wavelength-tuneable pump and probe pulses were generated in separate OPAs (Light Conversion Orpheus HP and Orpheus ONE).A combination of an optical delay stage and pulse picking from the dualamplifier (Light Conversion, Pharos, 15 W, 100 kHz, 260 fs output and Pharos SP, 6W, 100 KHz, 180 fs) laser system covered pump-probe time delays from < 1 ps to several microseconds.
The concentrations of the organic photocatalyst solutions used in TEAS and TVAS measurements were 1.0 -2.5 mM, with typical concentrations being 1.0 mM (2Cz-BP), 1.25 mM (2tCz-BP), 1.5 mM (2PTZ-BP) and 2.5 mM (2PXZ-BP and 4DP-IPN).Samples were prepared in dichloromethane (DCM, spectroscopy grade, 99.9%) or N,N-dimethyl ≥ formamide (DMF, spectroscopy grade, 99.9%) and were circulated through a stainless-≥ steel Harrick cell (200 m path length) fitted with CaF 2 windows using a peristaltic pump. Samples for TVAS were sparged with dry nitrogen gas to reduce the influence of quenching by dissolved oxygen.This sparging was not necessary for TEAS measurements with a maximum delay time of 1.3 ns.
Aggregation experiments for solutions of 2PTZ-BP were conducted using a Thermo Scientific Genesys 10S UV-Vis spectrometer, and an Edinburgh Instruments Spectrofluorometer FS5.In these measurements, 5 mL dilute solutions of 2PTZ-BP were prepared with various DMF to water ratios (by volume), and steady-state UV-Visible absorption spectra and fluorescence spectra (with a 360-nm excitation wavelength) were recorded using a 1 cm quartz cuvette.
Steady-state PL emission spectra at room temperature were obtained with a QuantaMaster 40 UV/vis steady state spectrofluorometer (Photon Technology International Inc.) equipped with a 75W Xe short arc lamp.The emission spectra were corrected for the sensitivity of the photomultiplier tube.Time-correlated single photon counting (TCSPC) experiments of 10 M OPC solutions were  conducted at Seoul National University to characterize prompt and delayed components of fluorescence.The excitation source was a 377 nm pulsed diode laser (LDH series PicoQuant) of pulse width (FWHM) < 49 ps.Prompt fluorescence decay measurements were carried out by a PicoHarp−300 TCSPC event timer (PicoQuant) with 64 ps time resolution.Soapy water was used to scatter excitation light for measurement of the instrument response function (IRF).In prompt fluorescence measurements, the samples were not sparged to eliminate any contribution of delayed fluorescence to the measurements.Delayed fluorescence measurements were carried out by a NanoHarp−250 TCSPC event timer after degassing dissolved oxygen by sparging for 10 mins with 99.9999% Ar gas.The decay time fitting procedure was carried out using Origin for prompt fluorescence emission, and the Fluofit software (PicoQuant) for delayed fluorescence emission.

S2.1 Experimental Details for Steady State Measurements
Steady state UV-visible absorption spectra were recorded using a PerkinElmer LAMBDA 950 UV-Vis spectrometer at the STFC Rutherford Appleton Laboratory.2.5 mM solutions of 2Cz-BP, 2tCz-BP, 2PTZ-BP and 2PXZ-BP were prepared in DCM, and a 2.5 mM solution of 4DP-IPN was prepared in acetonitrile (MeCN).Samples were introduced into a stainless-steel Harrick cell (200 m path length) fitted with 3 mm thick calcium fluoride (CaF 2 ) windows, and  a single spectrum was recorded over the range 200 -800 nm for each compound.Background spectra were recorded for pure solvent using the same Harrick cells as for sample acquisitions.

S2.3 Steady state photoluminescence emission spectroscopy
Photoluminescence (PL) emission spectra for 2Cz-BP, 2tCz-BP and 4DP-IPN measured in solvents of varying polarity are shown in Figure S7.These PL spectra demonstrate distinct solvatochromic shifts consistent with the assigned intramolecular charge-transfer (ICT) character of the electronically excited states.For 2Cz-BP in toluene the PL emission intensity is very low, so this spectrum has not been shown.The weak emission intensities observed for 2PTZ-BP and 2PXZ-BP are consistent with our assignment of these species as AIEgens.In DCM, DMF and THF, monomeric forms are favoured in solution and are only weakly emissive, so the recorded PL spectra are contaminated with Raman bands.However, because of their low solubility in toluene, aggregation causes appreciable emission.This aggregation induced emission typically shifts to longer wavelengths (500 -700 nm, as shown in the PL spectra in THF and toluene plotted below).However, in DMF the emission bands assigned to aggregates of 2PTZ-BP and 2PXZ-BP exhibit smaller red shifts relative to the monomer emission bands compared to other solvents.Emission bands assigned to aggregates in DMF span wavelengths from (410 -480 nm) and (410 -460 nm) for 2PTZ-BP and 2PXZ-BP, respectively.TCSPC measurements of 2PTZ-BP and 2PXZ-BP in DCM and DMF recorded using a 570 nm detection wavelength were compared to measurements made with a 404 nm detection wavelength (Table 1 and Table S5).Time constants for the decay traces were consistent regardless of detection wavelength, verifying that fluorescence emission is originating from the same species (aggregates), and the shifted emission bands in 2PTZ-BP and 2PXZ-BP can be attributed to aggregation.

S3 Natural Transition Orbital Diagrams for Key States
For carbazole-type OPCs, natural transition orbitals (NTOs) are only shown for S 2 ← S 0 locally excited (LE) electronic transitions.At the B97XD/6-31+G(d) level of theory, DFT fails to ω predict S 1 ← S 0 charge-transfer (CT) transitions observed at wavelengths greater than 350 nm, therefore the first calculated singlet state excitation ( = 321 nm) is representative of the  second electronic transition, not the first.Similarly, only the S 2 ← S 0 transition is shown for 2PTZ-BP due to the S 1 state being optically inaccessible.A full discussion is presented in the main text, section 3.1, together with NTO projections for 2PXZ-BP.DFT calculations also fail to predict the S 1 ← S 0 CT transition for 4DP-IPN arising between 475 and 500 nm, therefore only the S 2 ← S 0 CT transition is presented.

S4.2 Emission Wavelength Dependent TCSPC
To verify that fluorescence emission occurs from a single species, TCSPC experiments were performed for solutions of 2PTZ-BP and 2PXZ-BP in DCM and DMF.Fluorescence decay traces were recorded at three detection wavelengths (404 nm, 430 nm and 480 nm) for each sample, from which fluorescence lifetimes were determined.Detection wavelengths were selected to span most of the (very weak) fluorescence emission bands of 2PTZ-BP and 2PXZ-BP in these solvents.The fitted time constants are reported in Tables S5 -S8.As discussed in S2.3, time constants measured using a 570 nm detection wavelength are consistent with those measured using a 404 nm detection wavelength.This correspondence indicates that there is overlap between the emission bands of aggregates and monomers.These results show that the prompt and delayed fluorescence lifetimes are unchanged regardless of detection wavelength, and therefore support our interpretation that emission is occurring from a single species showing TADF behaviour.

S4.3 TCSPC for Aggregates of 2PTZ-BP and 2PXZ-BP
The observed photoluminescence (PL) emission intensities of 2PTZ-BP and 2PXZ-BP in neat DMF are much lower than for other OPCs studied.Therefore, to collect time-resolved fluorescence traces for these samples, the required energy of the excitation laser is between two and three times larger than that used for other OPCs.Acquisitions also require approximately ten times the photon counting of other samples, and the slit width of the detector is set to the maximum to enhance weak signals.On addition of water (90% by volume, FW=0.9) much greater PL emission intensity is observed from the samples.We attribute this enhanced luminescence to aggregation induced emission in this class of OPC.Furthermore, the delayed fluorescence lifetimes observed in neat DMF are approximately two to three times smaller than those in the mixed DMF / water solution, which is attributed to suppression of non-radiative decay pathways because of aggregation.

S5 Analysis of Transient Absorption Spectra
Transient electronic absorption spectra (TEAS) and transient vibrational absorption spectra (TVAS) were processed and analysed using the KOALA2 program. 9For all measurements, a flat-shift baseline correction was applied to the data, and a spectrum recorded at negative time delay (i.e., with the probe pulse preceding the pump pulse) was subtracted from transient spectra obtained at all subsequent time delays.Correction to TEAS measurements to account for chirp introduced by the broadband white-light continuum (WLC) probe pulse was performed during analysis, as implemented in KOALA2.However, chirp correction was not necessary for TVAS measurements.Spectra were decomposed into constituent transient absorption bands using a combination of Gaussian functions and basis spectra as described below.Time constants were obtained by fitting exponential functions to the time-dependent integrated band intensities of the various components of the decomposed spectra.This fitting was performed in Origin software.

S5.1 Decomposition of TEA Spectra
TEAS analysis of 2PTZ-BP in DCM used a combination of two Gaussian functions to model the evolution of the S 2 excited state absorption (ESA), and a basis spectrum corresponding to the TEAS measurement at a 50-ps time delay to follow the S 1 ESA feature.Figure S19 shows a series of analysis frames at representative times points to highlight how the Gaussian functions and 50-ps basis spectrum interact to yield an overall fit that matches the experimental data well.Spectral decomposition was limited to the range 375 -625 nm, therefore individual components of decomposition do not extend beyond this range, observed in figure S19 as a vertical cut-off in the total fit at longer wavelengths.
TEA spectra for carbazole-type OPCs (2Cz-BP and 2tCz-BP) were analysed by gating relevant transient features within a defined wavelength range, and then integrating across these regions at each time point to extract kinetics.Integration bands were selected to follow the hot S 1 ESA (approximately 400-425 nm), stimulated emission at early times assigned to vibrationally hot S 1 state (approximately 470-490 nm) and stimulated emission at late times assigned to the vibrationally cool S 1 state (approximately 495-515 nm).

S5.2 Decomposition of TVA Spectra Measured in DCM
For TVAS measurements acquired using the LIFEtime facility 2, 3 (section S1.1), the two ~ 200 cm -1 bandwidth IR probe pulses were dispersed and detected, calibrated and analysed independently.Where there is overlap of the probe regions observed by the left-hand and right-hand detectors (lhd and rhd) monitoring the two probe pulses, transient features were analysed only once using the detector for which the feature is better resolved.
Analysis frames at representative time points for photoexcited 2Cz-BP and 2PXZ-BP in DCM are shown in figures S20 and S21, respectively.Early time basis spectra (5 ps and 1 ps) were used to model bands corresponding to the S 1 electronic states of 2Cz-BP and 2PXZ-BP and have been modified such that any negative A values are instead set to zero.The purpose of this modification was to avoid mutual dependency between the S 1 ESA features and the GSB features, which were fitted using Gaussian functions.For carbazole-type species, ESAs assigned to T 1 states were also observed and were fitted using Gaussian functions.As discussed in the main text and Section S1, TVA spectra for all five OPCs in DMF were recorded over the range 1400-1700 cm -1 to observe ESA bands attributed to aromatic ring modes, as well as GSB features representative of the electronic ground state.DMF exhibits strong absorption bands in this IR region, therefore the spectra presented in figure S22 are restricted to the 1530-1610 cm -1 range.

BP and 2PTZ-BP (a-c), and a 425 nm pump pulse for 2PXZ-BP and 4DP-IPN (d-e). Spectra are coloured to indicate the delay time of the broadband IR probe pulse, and black arrows show the directions of changes of band intensity with time. (f-i) Kinetic traces for the photocatalysts obtained from TVAS measurements in DMF. Solid lines are global exponential fits to data points (closed circles). Fitted exponential time constants are presented in the main text (table 2).
TVAS data for carbazole-type OPCs show that the photophysics of this class of molecule are consistent between DCM and DMF solvents.In both solvents, the kinetics of 2Cz-BP and 2tCz-BP are well modelled using bi-exponential decay functions with a prompt lifetime ( ,   section 3.3 in the main text) and a delayed lifetime ( ) of 6 -7 ns and 1 -2 s, respectively,    in DMF.These lifetimes are comparable to the = 4 -8 ns and = 0.5 -2.5 s observed in      DCM.TVAS in DMF also shows direct evidence of triplet formation concurrent with S 1 state decay and partial GSB recovery on the timescale of the prompt lifetime components.Therefore, we assign the carbazole-type OPCs as TADF emitters in DMF using the same arguments as for these compounds in DCM in the main text.
Kinetics extracted from TVA spectra for 4DP-IPN in DMF also closely resemble those extracted in DCM, with ~ 3 ns in both solvents, and ~ 4 s and 2.7 s in DMF and DCM       respectively.Decomposition of the TVAS data for 4DP-IPN is presented in section S5. 4   TVAS experiments for 2PTZ-BP and 2PXZ-BP in DMF reveal kinetics similar to those in DCM.In both solvents we observe rapid mono-exponential decay of GSB features and ESA bands (assigned to the S 1 state) on sub-nanosecond timescales.Kinetic analysis of this class of OPC yields prompt lifetimes in DMF of 170 ps and 46 ps for 2PTZ-BP and 2PXZ-BP, which are shorter than the prompt lifetimes of 294 ps and 350 ps for 2PTZ-BP and 2PXZ-BP observed in DCM.

S5.4 Decomposition of TVA Spectra Measured in DMF
Decomposition of TVA spectra for 2Cz-BP, 2tCz-BP and 2PTZ-BP in DMF was approached in the same way as decomposition of analogous spectra in DCM (S5.2).For carbazole-type OPCs a modified early-time basis spectrum was used to represent the S 1 ESA features, and Gaussian functions were used to model the GSB features and emergent T 1 ESA.For 2PTZ-BP in DMF, the GSB features were each modelled with a Gaussian function (distinguished in figure S22, panel h, by their central wavenumber), and the evolution of the S 1 ESA was followed with a modified basis spectrum.
Figures S23 and S24 show the decompositions for 2PXZ-BP and 4DP-IPN respectively as these differ from methods used to decompose TVAS data in DCM solvent.Due to the restricted analysis region in DFM, 2PXZ-BP spectra were simply decomposed using two Gaussian functions representing the S 1 and S 0 state absorptions.4DP-IPN spectra were instead decomposed using a modified early-time basis spectrum (2 ps) for the ESA band, and four Gaussian functions for the GSB features.Because each fitted Gaussian function evolved with the same time constants (on account of each function modelling recovery of the S 0 electronic state population), the kinetics for each function are presented in the same colour in figure S22 panel j for clarity.

S5.5 Triplet Quantum Yields
As discussed in the main text, quantum yields of triplet formation for the OPCs can be estimated from TVA spectra by comparing the amplitudes of the prompt and delayed components of GSB recovery kinetics.

Where
is the quantum yield for T 1 formation, A P is the amplitude of the prompt GSB Φ( 1 ) recovery component, and A D is the amplitude of the delayed GSB recovery component.*Note that because estimates have been made using TVAS data, the values quoted for 2PTZ-BP and 2PXZ-BP are representative of the non-aggregated states.
# No evidence for triplet formation was observed in this measurement.

S7 O-ATRP performance for the organic photocatalysts
Table S12 summarizes ground and excited-state redox potentials for the five studied OPCs, their photochemical parameters, and performance as O-ATRP catalysts for the following polymerization reaction.
The scheme shows polymerization of MMA using OPCs with DBM as the initiator.Polymerizations were carried out in inert environments (N 2 ) at room temperature using In table 12, the polymerization parameters are defined as follows:  is the polymerization yield; Ɖ is the polymer dispersity; I* is the initiator efficiency, quantifying activation of the initiator by electron transfer.I* is defined as I * = M n,calc./M n,exp where M n is the number average molecular weight, 'calc.' stands for 'calculated' and 'exp.' stands for 'experimental'.The 'calculated' values are theoretical values from the yield (or conversion) under the assumption that all initiators are activated.The 'experimental' values are obtained by gel permeation chromatography.For example, if M n,calc is small and M n,exp is large, I * takes a small value indicating that only a small portion of the initiator was activated during the polymerization.a Determined gravimetrically.b Determined by gel permeation chromatography equipped with refractive index detector using PMMA standards.c I * is defined in the main text.d Carried out under the irradiation of two 3-W 515-nm LEDs.e Determined by TVAS measurement using OPC solutions in DMF in this work.f Determined via TCSPC measurements using OPC solutions in DMF in this work.

Figure S6
Figure S6 FTIR spectra (black lines) for solutions of OPCs in DCM overlaid with vibrational frequencies (red bars) calculated at the B97XD/6-31+G(d) level of theory.Calculated IR  frequencies are shown without any form of scaling applied.

Figure S7
Figure S7 Photoluminescence emission spectra for 10 M solutions of (a) 2Cz-BP, (b) 2tCz- BP, (c) 4DP-IPN, (d) 2PTZ-BP and (e) 2PXZ-BP obtained using an excitation wavelength of 360 nm.Samples were dissolved in four solvents (DCM, DMF, THF and toluene), and were not sparged prior to irradiation.Sharper features evident in some spectra are Raman bands.

Figure S9
Figure S9NTOs involved in the S 2 ← S 0 LE electronic transition for 2PTZ-BP.

Figure S10
Figure S10NTOs involved in the S 2 ← S 0 CT electronic transition for 4DP-IPN.

Figure
Figure S12 Time-resolved fluorescence decay traces for 10 M solutions of 2tCz-BP in DMF  (top row) and DCM (bottom row).Solid red lines are exponential fits to raw data (black circles and grey lines).Decay traces were obtained using a 377 nm excitation pulse and a detection wavelength of 530 nm.Samples for prompt fluorescence traces (left column) were not sparged, whereas samples for delayed fluorescence traces (right column) were sparged with argon gas for 10 minutes prior to irradiation.

Figure
Figure S13 Time-resolved fluorescence decay traces for 10 M solutions of 4DP-IPN in DMF (top row) and DCM (bottom row).Solid red lines are exponential fits to raw data (black circles and grey lines).Decay traces were obtained using a 377 nm excitation pulse and a detection wavelength of 530 nm.Samples for prompt fluorescence traces (left column) were not sparged, whereas samples for delayed fluorescence traces (right column) were sparged with argon gas for 10 minutes prior to irradiation.

Figure S14
Figure S14 Time-resolved fluorescence decay traces for 10 M solutions of 2PTZ-BP in DMF (top row) and DCM (bottom row).Solid red lines are exponential fits to raw data (black circles and grey lines).Decay traces were obtained using a 377 nm excitation pulse and a detection wavelength of 404 nm.Samples for prompt fluorescence traces (left column) were not sparged, whereas samples for delayed fluorescence traces (right column) were sparged with argon gas for 10 minutes prior to irradiation.

Figure S15
Figure S15 Time-resolved fluorescence decay traces for 10 M solutions of 2PXZ-BP in DMF  (top row) and DCM (bottom row).Solid red lines are exponential fits to raw data (black circles and grey lines).Decay traces were obtained using a 377 nm excitation pulse and a detection wavelength of 404 nm.Samples for prompt fluorescence traces (left column) were not sparged, whereas samples for delayed fluorescence traces (right column) were sparged with argon gas for 10 minutes prior to irradiation.

Figure S19
Figure S19 Example decomposition of TEAS data at several time delays from 0.3 -1100 ps for a solution of 2PTZ-BP in DCM photoexcited at 360 nm.Panels show the experimental TEAS data (black, solid line), the total fit (red, dashed line), Gaussian functions used to represent the S 2 state (blue and orange, dotted lines), and a basis spectrum acquired at 50 ps to represent the S 1 state ESA (pink, dash and dot line).

Figure S20
Figure S20Example decomposition of TVAS data at several time delays from 0.005 -5000 ns for a solution of 2Cz-BP in DCM photoexcited at 360 nm.Panels show the experimental TVAS data (black, solid line), the total fit (red, dashed line), Gaussian functions used to represent the T 1 state bands (blue, dotted line) and S 0 GSB features (pink, dash-dot-dot line), and a basis spectrum acquired at 5 ps to represent the S 1 state (orange, dash-dot line).

Figure S21
Figure S21 Example decomposition of TVAS data at several time delays from 0.001 -10 ns for a solution of 2PXZ-BP in DCM photoexcited at 425 nm.Panels show the experimental TVAS data (black, solid line), the total fit (red, dashed line), Gaussian functions to represent the S 0 GSB features (orange, dash-dot line) and a basis spectrum acquired at 1 ps to represent the S 1 state ESA (blue, dot line).

Figure S23
Figure S23 Example decomposition of TVAS data at several time delays from 1 -550 ps for a solution of 2PXZ-BP in DMF excited at 425 nm.Panels show the experimental TVAS data (black, solid line), the total fit (red, dashed line), and Gaussian functions used to represent the S 1 ESA feature (blue, dot line) and S 0 GSB feature (orange, dash-dot line).

Figure S24
Figure S24Example decomposition of TVAS data at several time delays from 0.002 -8000 ns for a solution of 4DP-IPN in DMF excited at 425 nm.Panels show the experimental TVAS data (black, solid line), the total fit (red, dashed line), a basis spectrum acquired at 2 ps used to represent the S 1 state (blue, dot line) and four Gaussian functions used to represent GSB features centred around 1540 cm -1 (orange, dash-dot line) and 1595 cm -1 (pink, dash-dot-dot line).